1. Field of the Invention
The present invention relates to an optically recordable, reproducible, erasable and rewritable information recording medium, a method for manufacturing the medium and a method for recording/reproducing information thereon.
2. Description of the Prior Art
Conventionally, for a phase changeable information recording medium, a multilayered film including a recording layer where a reversible phase change is caused between a crystalline state and an amorphous state is formed on a transparent disk substrate by sputtering or the like in the film formation process. The structure of the recording layer is amorphous after the film formation, and then the recording layer is subjected to a process for changing the entire surface of the recording layer from the amorphous state to the crystalline state by optical or thermal means (hereinafter, referred to as an initialization process). Thus, a phase changeable information recording medium is manufactured. (In the specification of the present invention, the thus formed amorphous state in the film formation process is referred to as xe2x80x9cas-depo amorphousxe2x80x9d to be distinguished from the amorphous state formed by quenching after melting by power laser irradiation as described below.)
In the phase changeable information recording medium, signals can be recorded or rewritten by irradiating the recording layer with a single laser beam while changing the power between high and low. When the recording layer is irradiated with a high power laser beam to be molten and then quenched, the recording layer becomes amorphous (recorded state). When the recording layer is irradiated with a low power laser beam to be warmed and then cooled gradually, the recording layer becomes crystalline (erased state). Thus, a recording mark on the order of several tenths Am (several 100 nm) is formed on the track. The signals are reproduced by utilizing the difference xcex94R(%) (xcex94R=|Rcxe2x88x92Ra|) between the reflectance Rc (%) of the medium when the recording layer is in the crystalline phase and the reflectance Ra (%) of the medium when the recording layer is in the amorphous phase. In either the medium in which Rc greater than Ra or Ra greater than Rc, signals can be recorded/reproduced.
In the initialization process, the reflectance of the medium changes from Ra to Rc. In particular, in the medium optically designed to achieve Ra greater than Rc, the reflectance is reduced so that it is preferable that Rc is 10% or more.
The initialization process requires equipment provided with optical or thermal means. For example, in the case where a semiconductor laser is used as the optical means, operations for optimizing various conditions such as the shape of the laser beam, the power of laser irradiation, the cooling rate, the rotational speed of the medium and the period of time for irradiation with respect to each particular medium are required. In addition, other problems arise. For example, it is known that the volume of the recording layer is contracted by several % at the time of the phase change from the amorphous phase to the crystalline phase. Therefore, when the recording layer is crystallized after the multilayered film is formed, the volume contraction of the recording layer generates new internal stress, which was not present immediately after the film formation, at least in the layer in contact with the recording layer. If the recording layer is as thin as 10 nm or less, light absorption is small and heat is diffused readily, so that crystallization requires more power density so that a load is applied to grooves or address pits that previously have been transferred on the substrate. Thus, the initialization process poses a large number of problems.
If the initialization process is eliminated, the plant investment and the development cost can be reduced, leading to a significant reduction in the cost of the medium. Different systems to eliminate the initialization process can be conceived for (1) the medium of Rc greater than Ra and (2) the medium of Ra greater than Re. In order to obtain good servo characteristics, it is preferable to keep the reflectance high, and it is required that in the case of (1), the recording layer is in the crystalline phase (initial state Rc) after the film formation, and that in the case of (2), the recording layer is in the amorphous phase (initial state Ra) after the film formation. Herein, the initial state refers to the state of the medium before recording. In order to meet these requirements, a technique to crystallize the recording layer during the film formation and a technique to record signals in an amorphous recording layer are required.
A method for crystallizing a recording layer of a phase changeable optical information recording medium during the film-formation is disclosed in WO98/47142. In this method, a crystallization accelerating layer made of a material whose crystal structure is face-centered cubic lattice or rhombohedral lattice is provided, and then the recording layer is formed directly on the crystallization accelerating layer and the substrate temperature is changed from 45xc2x0 C. to 110xc2x0 C. during the formation of the recording layer. Furthermore, the examples show that the crystallization accelerating layer is formed of a material comprising at least one of Sb, Bi and Sb compounds and Bi compounds, and the recording layer of the phase changeable optical information recording medium manufactured by this method is formed in the crystalline state.
Furthermore, PCT International Publication No. WO98/38636 discloses methods for manufacturing a phase changeable optical information recording medium that is designed to attain Ra greater than Rc. In this disclosure, a method in which the substrate temperature is from 35xc2x0 C. to 150xc2x0 C. during formation of a recording layer, and a method in which the substrate temperature is from 35xc2x0 C. to 95xc2x0 C. immediately before formation of the recording layer are described. The thus produced phase changeable optical information recording medium can achieve high recording characteristics, even if recording is performed first on the as-depo amorphous recording layer without performing an initialization process.
However, in WO98/47142, Bi has a melting point as low as about 271xc2x0 C., so that it is impossible to raise sputtering power. In WO98/38636, in order to form a film having an as-depo amorphous recording layer by heating the substrate, the entire surface of the substrate is heated uniformly and the temperature is required to be kept. For example, when heating the substrate holder itself, it is very difficult to heat the entire substrate uniformly without contacting the entire surface of the substrate with the substrate holder so as to conduct heat to the substrate. However, when the substrate is contacted with the holder on its entire surface, scratches or dirt are likely to be generated on the surface of the substrate. In addition, when high frequency induction or flash heating is performed, complicated film-forming equipment is required in order to heat the substrate uniformly in a contactless manner in a vacuum apparatus. Moreover, it is difficult to keep a constant temperature stable immediately before or during formation of the film. Furthermore, it is necessary to measure the temperature of the substrate in a contactless manner in the vacuum apparatus and to monitor the temperature outside the apparatus, so that the apparatus inevitably becomes complicated and largescale.
It is believed that the reason why it conventionally is difficult to perform recording on an as-depo amorphous phase is that the as-depo amorphous phase is different in nature from the amorphous phase formed by irradiating a crystalline phase with laser. In general, the amorphous phase has several metastable energy states. When a medium is stored for a long time or under high temperature conditions, the energy state can be changed to a different energy state after the storage. For this reason, since optimal conditions for recording/reproducing are different between before and after the storage, the recording/reproducing characteristics can be changed when recording/reproducing is performed under the same conditions. For example, when a recording mark in the recording layer is shifted to be in a more stable energy state, sensitivity for erasure by crystallizing the recording layer is reduced, so that the erasure ratio can be dropped at the time of overwriting information signals.
Therefore, with the foregoing in mind, it is a first object of the present invention to provide an information recording medium formed of a material having a high melting point where the recording layer is in a crystalline phase when the film formation is complete without warming the substrate, and thus does not require an initialization process, and a method for manufacturing the same, and to provide an information recording medium that requires reduced energy for crystallization.
It is a second object of the present invention to provide an information recording medium that does not require precise control of the temperature of the substrate immediately before or during film-formation and that allows a recording operation to be performed on the recording layer in the as-depo amorphous state of the information recording medium optically designed to satisfy Ra greater than Rc, without the initialization process.
It is a third object of the present invention to provide an information recording medium that does not require the initialization process and allows stable reading of addresses or tracking servo control even if Rc is substantially 0%, and to provide a method for manufacturing the information recording medium and a method for recording/reproducing information thereon.
In order to achieve the above objects, an information recording medium of the present invention includes at least a recording layer formed on a substrate, the recording layer including a phase change layer in which a reversible phase change is caused between a crystalline state and an amorphous state by irradiation of a light beam, and a crystallization-ability improving layer for improving the crystallization ability of the phase change layer. The crystallization-ability improving layer is formed before forming the phase change layer. Thus, crystal nucleus generation and crystal growth are caused during formation of the phase change layer, so that at least a portion of the phase change layer is in the crystalline phase after the formation. In this embodiment, it is preferable that the crystallization-ability improving layer is formed of at least one selected from the group consisting of a telluride and a halogenide. More specifically, it is desirable that the telluride is at least one selected from the group consisting of SnTe, PbTe, Te, Sb2Te3, Bi2Te3, GeSbTe eutectic and GeBiTe eutectic, and that the thickness thereof is from 1 nm to 10 nm. It is desirable that the halogenide is at least one selected from the group consisting of ZnF2, AlF2, KF, CaF2, NaF, BaF2, MgF2, LaF3, and LiF, and that the thickness thereof is from 1 nm to 20 nm. It is preferable that the phase change layer is formed of a material comprising GeSbTe as the main component and having a halite type crystal structure. It is more preferable that also the crystallization-ability improving layer has a halite type crystal structure. The phase change layer is formed preferably at a rate r (nm/min) in a range from 5 nm/min to 20 nm/min. It is possible to make the formed phase change layer be in the crystalline phase by using a telluride and a halogenide as the material for the crystallization-ability improving layer and forming the phase change layer at a low rate.
Furthermore, the present invention has a function to achieve A less than B, where A is an energy for crystallizing the phase change layer in the case where the crystallization-ability improving layer is formed, and B is an energy for crystallizing the phase change layer in the case where the crystallization-ability improving layer is not formed.
According to another aspect of the present invention, an information recording medium of the present invention is a two layered information recording medium formed by attaching a first information recording medium comprising at least a first recording layer formed on a first substrate and a second information recording medium comprising at least a second recording layer formed on a second substrate. The first recording layer includes a phase change layer in which a reversible phase change is caused between a crystalline state and an amorphous state by irradiation of a light beam, and a crystallization-ability improving layer for improving the crystallization ability of the phase change layer. The crystallization-ability improving layer is formed before forming the phase change layer, so that crystal nucleus generation and crystal growth are caused during formation of the phase change layer, and at least a portion of the phase change layer is in the crystalline phase after the formation. This embodiment allows crystal nucleus generation and crystal growth during formation of the first recording layer, so that at least a portion of the phase change layer is in the crystalline phase after the formation.
Furthermore, the present invention is an information recording medium including a recording layer on a substrate. The recording layer includes a phase change layer in which a reversible phase change is caused between a crystalline state and an amorphous state by irradiation of a light beam (herein, xe2x80x9clayerxe2x80x9d refers not only to a layer formed uniformly all over, but also a layer formed in a shape of an island, which also applies to a crystal nucleus supplying layer); and a crystal nucleus supplying layer that is laminated on the phase change layer and accelerates crystallization of the phase change layer. The information recording medium of this embodiment allows recording to be started on the phase change layer in the as-depo amorphous state. Furthermore, the information recording medium of this embodiment provides a highly reliable information recording medium for recording/reproducing information signals at a high density and a high linear velocity.
In the information recording medium, it is preferable that the crystal nucleus supplying layer and the phase change layer are formed from a substrate side in this order. It is preferable that the information recording medium further includes a second crystal nucleus supplying layer for accelerating crystallization of the phase change layer, and the phase change layer and the second crystal nucleus supplying layer are formed from a substrate side in this order. It is preferable that the phase change layer and the crystal nucleus supplying layer are formed from a substrate side in this order.
In the information recording medium, it is preferable that the transition temperature Tx1 (xc2x0 C.) from the amorphous phase to the crystalline phase of the crystal nucleus supplying layer (hereinafter, referred to crystallization temperature) and the crystallization temperature Tx2 (xc2x0 C.) of the phase change layer satisfy the relationship: Tx2 greater than Tx1. This embodiment facilitates the crystallization of the phase change layer. In the information recording medium, it is preferable that the melting point Tm1 (xc2x0 C.) of the crystal nucleus supplying layer and the melting point Tm2 (xc2x0 C.) of the phase change layer satisfy the relationship: Tm1 greater than Tm2. This embodiment provides an information recording medium in which the crystal nucleus supplying layer is highly stable even if the crystal nucleus supplying layer is provided nearer the laser beam incident side than the phase change layer is.
In the information recording medium, it is preferable that the crystal nucleus supplying layer comprises Te. This embodiment facilitates the crystallization of the phase change layer because Te functions as the crystal nucleus. In the information recording medium, it is preferable that the crystal nucleus supplying layer comprises at least one selected from the group consisting of SnTe and PbTe. In the information recording medium, it is preferable that the crystal nucleus supplying layer comprises SnTexe2x80x94M, where M is at least one selected from the group consisting of N, Ag, Cu, Co, Ge, Mn, Nb, Ni, Pd, Pt, Sb, Se, Ti, V Zr and PbTe). Herein, SnTexe2x80x94M is SnTe provided with M without changing the ratio of Te that is present with respect to the Sn that is present. For example, SnTexe2x80x94M includes compounds of SnTe and M and eutectics of SnTe and M. The content of the M is preferably at most 50%, more preferably 0.5-50 atom %. Furthermore, a preferable composition of SnTe is the stoichiometric composition of Sn50Te50 (Sn:Te=50:50), but a tolerance of about xc2x15% such as Sn45Te55 (Sn:Te=45:55), and Sn55Te45 (Sn:Te=55:45) is possible.
In the information recording medium, it is preferable that the phase change layer is formed of a chalcogen based material. This embodiment provides an information recording medium on which information can be recorded at a high density. In the information recording medium, it is preferable that the phase change layer comprises at least one selected from the group consisting of GeTe, GeSbTe, TeSnSe, InSbTe, GeBiTe and AgInSbTe. In the information recording medium, it is preferable that the phase change layer comprises GeSbTe and at least one element selected from the group consisting of Ag, Sn, Cr, Mn, Pb, Bi, Pd, Se, In, Ti, Zr, Au, Pt, Al and N.
In the information recording medium, it is preferable that the thickness d1 (nm) of the crystal nucleus supplying layer and the thickness d2 (nm) of the phase change layer satisfy the relationship: d2 greater than d1. This embodiment prevents the amount of a laser beam incident on the phase change layer from being insufficient. In the information recording medium, it is preferable that the thickness d1 (nm) of the crystal nucleus supplying layer is in the range of 0.3 less than d1xe2x89xa65. In the information recording medium, it is preferable that the thickness d2 (nm) of the phase change layer is in the range of 3xe2x89xa6d2 greater than 20
In the information recording medium, the reflectance Rc (%) of the information recording medium when the phase change layer is in the crystalline phase and the reflectance Ra (%) of the information recording medium when the phase change layer is in the amorphous phase satisfy the relationship: Ra greater than Rc. This embodiment provides an information recording medium in which grooves or addresses formed on the substrate can be detected easily.
According to another aspect of the present invention, a method for manufacturing an information recording medium of the present invention, the information recording medium comprising at least a recording layer, includes forming the recording layer. The recording layer includes a phase change layer in which a reversible phase change is caused between a crystalline state and an amorphous state by irradiation of a light beam; and a crystal nucleus supplying layer that is laminated on the phase change layer and accelerates crystallization of the phase change layer. The method for manufacturing an information recording medium of this embodiment allows the information recording medium of the present invention to be produced easily. In the method for manufacturing an information recording medium, it is preferable that the step of forming the phase change layer is performed under a condition that allows the phase change layer to become amorphous. This embodiment allows as-depo recording.
In the method for manufacturing an information recording medium, the rate r(nm/min) at which the phase change layer is formed is preferably in the range of rxe2x89xa730. This embodiment allows the formed phase change layer to be in the amorphous state.
According to another aspect of the present invention, the present invention provides a method for recording/reproducing information on an information recording medium, the information recording medium comprising at least a recording layer. The recording layer includes a phase change layer and in which a reversible phase change is caused between the crystalline state and the amorphous state, and a crystal nucleus supplying layer that is laminated on the phase change layer and that facilitates the crystallization of the phase change layer. The information is recorded by causing the phase change in the phase change layer by irradiating the recording layer with a laser beam. The method for recording/reproducing information on an information recording medium of this embodiment allows information to be recorded reliably.
In the method for recording/reproducing information on an information recording medium, it is preferable that the crystal nucleus supplying layer comprises at least one selected from the group consisting of SnTe and PbTe. This embodiment allows information to be recorded particularly reliably.
In the method for recording/reproducing information on an information recording medium, it is preferable that the phase change layer comprises at least one selected from the group consisting of GeTe, GeSbTe, TeSnSe, InSbTe, GeBiTe and AgInSbTe. This embodiment allows information to be recorded particularly reliably.
In the method for recording/reproducing information on an information recording medium, it is preferable that the formed phase change layer is in the amorphous state, and recording information is started on the phase change layer in the amorphous state without the phase change layer being crystallized.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.